The present invention provides an improved means of driving a chart recorder pen so that it rapidly and accurately follows the magnitude of an input electrical signal being charted by the recorder. In the last several years the advent of analog to digital conversion systems has made it practical to convert an analog input signal into a digital number or count. The pen carriage is then moved across the chart a distance equivalent to this count thus making unnecessary the slidewire or other mechanical reference devices of the earlier analog systems. The usual motive power for the digital pen drive is a stepper motor, a motor that receives counted pulses of electrical power and moves through a predetermined angle for each pulse.
Recorder pen drives using stepper motors for this purpose have been limited by the nature of the motor to either low slewing speeds or low resolution in pen location. The stepper motor is large, noisy, expensive, runs hot even when stationary and is generally limited to around a maximum of four or five hundred steps per second. When driven by pulses from a circuit which generates and counts pulses to correspond to the magnitude of the input signal being recorded, the motor's rotation accurately follows the pulse count providing the rate is not too fast for the motor to respond. Should the rate be excessive or should the motor be obstructed or moved when not being pulsed, the pen location is lost.
The disadvantages of the stepper motor as a pen driving means can be avoided by using a lighter, less expensive, more efficient direct current motor preferably of the permanent magnet field type. This motor is not limited by principle to a maximum counting rate. The motor, per se, has no counting means, therefore, some counting or encoding device must be also driven by the motor. This has, in most encoder applications, comprised a disc with a multiplicity of circular rings having one, two, four, etc. sectors. This encoder disc is mounted on the shaft whose angular rotation is to be monitored. Contacts or optical pickups adjacent this disc would generate a digital number series as the disc revolved thus giving a digital count. As an encoder this device is effective but expensive and required a complex pickup system.
Therefore, it is an objective of the invention to provide the improvement of the direct current motor drive with a greatly simplified encoder disc and pickup system with both digital magnitude and direction of count being determined by a microprocessor sub-routine, thus eliminating the earlier expensive requirement of digitizing at the encoder disc.
A further objective is to provide an additional servo sub-routine to control the motor speed and direction of rotation through the same microprocessor and to provide error-rate damping control to achieve a high slew rate with negligible overshoot.
Yet another objective is to initiate these sub-routines by interrupt requests to a basic, low cost, multipurpose microprocessor which is also available to perform computation, range control, and other tasks set forth in related patent applications previously listed, these to be accomplished in the interim between interrupt requests.
A further objective is to provide a counter system which retains knowledge of the pen location irrespective of external physical displacement of the pen or obstruction of pen movement.
Another objective is to provide a simple automatic initialization procedure through the microprocessor for the counter system when the instrument is first turned on.
Another objective is to provide a software routine through the same microprocessor which applies a digital number generated by manual means to offset zero pen position on the chart as desired.
Yet another objective is to provide another coacting subroutine through the same microprocessor to prevent the pen from forcibly hitting mechanical end stops if excessive input is applied.